Charge pump and auxiliary pump for hydrostatic transmissions and integrated hydrostatic transaxles

ABSTRACT

A hydrostatic transmission. The hydrostatic transmission generally comprises a sump containing hydraulic fluid, a charge pump in flow communication with the sump for creating high pressure hydraulic fluid from the hydraulic fluid, a center section having hydraulic porting formed therein, a hydraulic pump, a hydraulic motor drivingly connected to the hydraulic pump through the hydraulic porting, and a manifold defining a gallery for storing the high pressure fluid. The gallery is in flow communication with the charge pump and the hydraulic porting.

RELATED PATENTS

This application is a Continuation of application Ser. No. 08/700,933filed Aug. 23, 1996 now U.S. Pat. No. 5,819,535, which is aContinuation-In-Part of U.S. application Ser. No. 08/451,162, filed May26, 1995, now U.S. Pat. No. 5,557,931, which is a Continuation of U.S.application Ser. No. 08/394,144, filed Feb. 24, 1995, now U.S. Pat. No.5,555,727.

BACKGROUND OF THE INVENTION

This invention relates to charge pumps and other auxiliary pumps used onhydrostatic transmissions (“HST”) for use in light duty applications.Such HSTs can either have their own housing and be attached to an axledriving apparatus, or can be incorporated within a housing that includesthe components of the axle driving apparatus. A unit that contains anHST within the housing of an axle driving apparatus is often referred toas a integrated hydrostatic transaxle (“IHT”). The operation of suchunits are described in U.S. Pat. Nos. 5,201,692 and 5,314,387, the termsof which are incorporated by reference.

SUMMARY OF THE INVENTION

This invention presents a unique and novel manner of providing andmounting charge pumps and auxiliary pumps for both IHTs or stand-aloneHSTs that are mounted to a separate transmission. As described in the'692 patent, an HST generally comprises a pump that receives and isrotated by an input shaft driven by a vehicle engine. The pump includesa plurality of pistons that contact a swashplate to cause axial movementthereof when the pump rotates. The pump is hydraulically connected to amotor, which is similar in construction to the pump. The motor receiveshydraulic fluid from the pump, and movement of the motor pistons againsta swashplate causes rotation of the motor, which is connected to anddrives an output shaft. The hydraulic connection between the pump andmotor is a closed circuit; however, in any such circuit there will bedeliberate and incidental leakage due to lubrication requirements, thehigh pressure of the hydraulic fluid and manufacturing tolerances. Thus,the HST requires a mechanism to replace fluid leaked from the closedcircuit. This replacement fluid is commonly called make-up fluid.

In present HST designs, the pump and motor are often mounted on a centersection that includes the hydraulic circuit therein. The hydrauliccircuit includes two sides: a high pressure side and a low pressureside. The low pressure side is sometimes referred to as the vacuum side.These two sides are reversed when the vehicle motion is changed fromforward to reverse.

Typically, the center section is mounted in a housing, and the housingprovides a hydraulic fluid sump. Make-up fluid is brought from the sumpinto the low pressure side of the hydraulic circuit to replace fluidwhich is lost therefrom due to leakage. Specifically, check valvesmounted directly into the center section or mounted in a separate platethat is in communication with the center section, as shown in the '692patent, provide a fluid flow path between the sump and the hydrauliccircuit. However, this arrangement often does not provide sufficientfluid flow into the low pressure side of the circuit to replace the lostfluid. Therefore, a charge pump may be used to assist in this process.In addition, the use of an auxiliary pump to supply pressurizedhydraulic fluid for various purposes is generally known. This inventionprovides for an efficient manner of mounting a charge pump and, in somecases, an auxiliary pump, external to the housing but still in directcommunication with the hydraulic circuit in the center section.

The external charge pump confers significant benefits with respect tothe accessibility on the hydraulic system design and configuration. Theexternal nature of the charge pump allows direct access via a simplehydraulic fitting to hydraulic fluid that can be used for auxiliaryfunctions. Internal charge pumps, in comparison, generally require acomplex series of chambers, connections and fittings in order for fluidto be accessible exterior to the housing. External pumps provideaccessibility without unit disassembly, thereby allowing replacement,addition or upgrade of a charge or auxiliary pump. Previousconfigurations were not accessible without disassembly of the HST orIHT.

Additional benefits and features of this invention will be disclosed inthe description of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view of a charge pump and IHT in accordancewith the present invention;

FIG. 2 is a sectional end view of the IHT shown in FIG. 1;

FIG. 3 is a partial sectional side view of the porting plate and chargepump of the IHT shown in FIG. 1, with a typical hydraulic fluid flowpath shown;

FIG. 4 is a partial sectional bottom plan view of the IHT shown in FIG.1, with the lower housing and charge cover partially cut away;

FIG. 5 is a sectional side view of the IHT shown in FIG. 1;

FIG. 6 is a sectional side view of another embodiment of an IHT inaccordance with the present invention, with the center section enclosedin the housing and porting plate mounted to the exterior of the housing;

FIG. 7 is a sectional side view of another embodiment of an IHT inaccordance with the present invention, with the porting plateincorporated as a part of the lower housing;

FIG. 8 is an elevational end view of an HST incorporating a charge pumpin accordance with the present invention;

FIG. 9 is a further elevational end view of the HST shown in FIG. 8;

FIG. 10 is a sectional side view of the HST shown in FIG. 9 along theline 10—10;

FIG. 11 is a partial elevational side view of the HST lower housing,porting plate and charge cover shown in FIG. 8;

FIG. 12 is a sectional side view of another embodiment of an HST with acharge pump in accordance with the present invention incorporating theporting plate into the lower housing;

FIG. 13 is a partial, sectional side view of the lower housing of theembodiment shown in FIG. 12;

FIG. 14 is a sectional side view of an IHT incorporating an auxiliarypump in accordance with another embodiment of the present invention;

FIG. 15 is a sectional end view of the IHT shown in FIG. 14;

FIG. 16 is a sectional side view of the auxiliary pump used inconnection with the IHT shown in FIG. 14 with a typical fluid pathshown;

FIG. 17 is a partial sectional bottom plan view of an IHT as shown inFIG. 14, with the lower housing, porting plate and auxiliary pump coverpartially cut away;

FIG. 18 is a hydraulic schematic of the embodiment as shown in FIG. 14;

FIG. 19 is a bottom plan view of an HST incorporating an auxiliary andcharge pump of the present invention, with a partial sectional view ofthe auxiliary pump cover;

FIG. 20 is a sectional side view of an HST incorporating an auxiliaryand charge pump embodiment of the present invention;

FIG. 21 is a hydraulic schematic of an embodiment that incorporatesseparate charge and auxiliary pumps;

FIG. 22 is a sectional side view of a further embodiment of an IHTincorporating separate charge and auxiliary pumps;

FIG. 23 is a sectional side view of a further embodiment of theinvention incorporating a gallery forming manifold;

FIG. 24 is a top view of the manifold shown in FIG. 23;

FIG. 25 is a bottom view of the manifold shown in FIG. 23;

FIG. 26 is a sectional side view of a further embodiment of theinvention incorporating a gallery forming manifold; and

FIG. 27 is a sectional side view of a further embodiment of theinvention incorporating a gallery forming manifold.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A charge pump embodiment of the invention incorporated in an IHT willfirst be described in accordance with the accompanying drawings. FIGS. 1through 5 illustrate an IHT configured with a horizontally split housingwith upper housing 21 and lower housing 22. The invention does riotrequire any specific housing configuration, and all known configurationscan be accommodated. All specifics of an IHT are not shown in thesefigures as the general operating of an IHT is known in the art.

Pump 24 is disposed on center section 26 and receives input shaft 25,which communicates with and is driven by a vehicle engine (not shown).Center section 26 includes internal porting 30 that hydraulicallyconnects pump 24 and motor 40. Pump pistons 23 engage adjustableswashplate 27 to create pressure within a center section internalporting 30. Housings 21 and 22 form a sump or reservoir 32 external tocenter section 26. Motor 40 is connected to and drives output shaft 41,which in turn drives gear 42 a, which is connected to gear 42 b, whichthen drives gear 43 and the remaining gears of differential 44.Differential 44 is in turn operatively connected to the output driveaxles 45 a and 45 b of the vehicle. The specifics of the output gearingand differential are not essential to this invention and are disclosedin the '387 patent.

The charge pump includes an inlet porting plate 28 external to the lowerhousing 22 at the external surface thereof, i.e., attached to the lowerhousing 22. Porting plate 28 is mounted so that its interior surface isin close proximity to center section 26. Gerotor housing 29, which isalso known as a charge cover, is attached to the external surface ofinlet porting plate 28, and gerotor set 34 is held in position by theconfiguration of input shaft 25. This embodiment also includes mountingthe porting plate 28 on other housing elements, or parts that may belabeled by other nomenclatures, that would perform functions similar tothe described upper and lower housings. Porting plate 28 includes aninlet 36 that is connected to an external reservoir 50 that containshydraulic fluid and which is typically mounted on a supporting member ofthe vehicle in which the IHT is mounted. Inlet 36 could also be incommunication with and receive fluid from sump 32 instead of reservoir50.

Input shaft 25 extends through pump 24 and center section 26 to drivegerotor set 34, which is of a standard design known in the art. Rotationof gerotor set 34 by shaft 25 creates a low pressure or suction at inlet36 that remains constant regardless of the forward or reverse status ofthe HST. The porting plate 28 also includes fluid passage 31 incommunication with gerotor set 34. The action of the gerotor will causefluid forced from the gerotor set into passage 31 to become pressurized.Thus, fluid is drawn through inlet 36 by the suction created by rotationof gerotor 34; the pressurized fluid is then forced into passage 31 andthen into gallery 37. Gallery 37 is formed and defined by the interfacebetween the inlet porting plate 28 and the center section 26. In theembodiment shown in FIG. 1, lower housing 22 includes an opening for thebottom surface of center section 26 to be approximately flush with theexternal bottom surface of lower housing 22. The attachment location ofcenter section 26 in upper housing 21 establishes the position of thecenter section 26. Other embodiments with different split lines to thehousing elements would have a similar opening. The positioning of theinterface between the center section and the porting plate is notcritical. In the embodiment shown in FIG. 1, the interface is flush withthe external housing, but may move from that position in otherembodiments.

In an alternative embodiment shown in FIG. 6, center section 26 could bemounted entirely within the IHT housing and inlet porting plate 28 wouldthen be mounted on the external surface of lower housing 22. The chargepump is then connected to internal porting 30 of center section 26 witha lower housing passage 96 and a gallery formed inside the lower housingby a gallery housing 97.

FIG. 7 shows another embodiment of this invention, similar to that shownin FIG. 6, wherein the porting plate has been integrally formed with thelower housing 22. Center section 26 is seated on internal surface 99 oflower housing 22. Gallery 37 for the hydraulic fluid is formed anddefined by the lower surface of center section 26 and the internalsurfaces of the integrally formed porting plate, i.e., lower housing 22.An o-ring 73 is used to aid in positioning and seating these elementsand prevents leakage of hydraulic fluid from gallery 37. The principalbenefit of this embodiment is the reduction in complexity by theelimination of a separate porting plate. In the embodiment shown in FIG.1 o-ring 73 seals gallery 37 from the interface between porting plate 28and lower housing 22 as well as from sump 32. With the integration ofporting plate and lower housing 22 into a single piece, the requirementto seal between these two is eliminated, which thus increases thereliability of the IHT.

In the embodiment shown in FIGS. 1-5, check valves 39 a and 39 b areoperatively connected to internal porting 30 of center section 26. Asshown in this embodiment, check valves 39 a and 39 b may be included inplugs that are threaded directly into the surface of center section 26.Alternatively, they may be located in a separate plate that is thensecured to center section 26.

The two check valves 39 a and 39 b each communicate with a differentside of the internal hydraulic porting 30. As previously stated, thehydraulic circuit, including internal hydraulic porting 30, consists ofa high pressure and a low pressure side, which are reversed when thevehicle direction, of motion is changed. Thus, while the HST is in theforward position, one side of the hydraulic circuit is under highpressure and the corresponding check valve is closed. At the same timethe other side of the hydraulic circuit is under low pressure and thatcorresponding check valve is open and receives hydraulic fluid fromgallery 37. When the HST is moved to reverse, the two sides switch andthe formerly low pressure side is under high pressure and thecorresponding check valve is now closed and the high pressure side isnow under low pressure, closing that corresponding check valve.

When the HST is in the neutral position, there is no demand for make-upfluid. However, since the rotational speed of input shaft 25 does notvary, the fluid pressure supplied to gallery 37 from gerotor set 34 willremain constant, requiring fluid to be diverted back to sump 32 throughrelief passage 38 formed into center section 26. Relief valve 35, whichmay be located at any point in passage 38, controls the flow throughpassage 38. While the relief passage was selected to be through thecenter section in this embodiment of the invention, alternative passagescan be formed through the porting plate 28 or through the housing 22;the locations of the relief passage in other than center section 26 areconsidered obvious variations of the invention. Also, relief valve 35can be a variety of devices in configurations that are known in the art,and would remain in communication with passage 38 regardless of thelocation of that passage.

A charge pump embodiment of the invention incorporated in an HST willnow be described in accordance with the accompanying drawings. Asdiscussed above, another embodiment of this invention uses the chargepump in a stand-alone HST that is mounted within a housing separate fromthat of the components that transfer power from the HST to one or moreaxle shafts. In such an embodiment, the specifics of the charge pump andrelated structures shown in FIG. 3 can remain the same. FIGS. 8-11 showsuch a stand alone HST unit incorporating a charge pump embodiment ofthe invention. The general operation of the HST is identical to thatdescribed above and elements that are structurally identical to thosedescribed above are given identical numerals.

As shown in FIGS. 9 and 10, HST 60 includes upper housing 61 and lowerhousing 62. In these figures, the rotation of motor 40 caused by actionof the motor pistons 45 against fixed swashplate 46 drives output shaft41. Output shaft 41 is supported by bearings 63, and extends through thetransmission mounting plate 68 that is integrally formed with upperhousing 61 so as to engage a separate axle driving apparatus (notshown).

As shown is FIG. 10, inlet 36 receives fluid from the housing for theaxle driving apparatus to which the HST 60 is mounted. Fluid may also bereceived from a reservoir 50 that would be mounted to the supportstructure of the vehicle in which the HST and axle driving apparatuswere attached. As in the IHT configuration, inlet 36 could also be incommunication with sump 32 formed by the upper housing 61 and lowerhousing 62.

Another embodiment of the charge pump and lower housing is shown inFIGS. 12 and 13, wherein the inlet porting plate 28 is integrally formedwith the lower housing 62, similar to the embodiment shown in FIG. 7.The explanation provided above for the IHT version of this embodiment isapplicable for the HST. The same benefits that accrue to the IHT fromthis embodiment also accrue to the HST.

An auxiliary pump embodiment of the invention incorporated in an IHT orHST will be described in accordance with the accompanying drawings. Afurther embodiment encompasses the use of a pump that provides hydraulicfluid to power attachments and implements, known in the art as anauxiliary pump, in addition to the charge pump with an IHT or with astand-alone HST that is mounted with a separate axle driving apparatus.Auxiliary pumps for use with HSTs are generally known in the art and areused to create hydraulic fluid flow to power attachments and implementssuch as a hydraulic mower deck lift. FIGS. 14 through 18 show an IHT 120with a charge pump and auxiliary pump mounted thereon in accordance withthis invention. The general operation of IHT 120 in this embodiment issubstantially identical to that of the IHT depicted in FIGS. 1-5, andsimilar parts are indicated with the same numeral preceded by thenumeral “1”. For the sake of brevity, such parts will not be discussedherein except as such specifically relates to this 20 embodiment of theinvention,

As discussed above, the auxiliary pump and charge pump combinationdisclosed herein can be used with an IHT, as shown in FIGS. 12-17, andcan also be used with an HST as shown in FIGS. 19 and 20 that has itshousing attached to a separate axle driving apparatus (not shown). Thegeneral operation of the auxiliary pump and charge pump in FIGS. 19 and20 is identical to that shown in FIGS. 12-17 and the followingdiscussion is applicable to both applications of this embodiment. Themounting of the HST housing through transmission mounting plate 168 isthe same as was described for FIG. 10 above.

As shown in FIG. 14, center section 126 is attached to upper housing 121such that the lower surface is generally at the same level as theexternal surface of lower housing 122. The center section 126 may be butneed not be attached to the surface of lower housing 122 in thisembodiment, but is positioned by the hole in the lower housing 122through which the center section 126 extends. Inlet porting plate 128 isattached directly to lower housing 122. Center section 126 includes twocheck valves 138 that, as per the preceding discussion, may be directlypressed into the bottom of the center section or which may beincorporated in a separate plate. Gerotor housing 129 is mounted toporting plate 128 and houses gerotor set 134. Auxiliary pump cover 90 isthen mounted to gerotor housing 129. O-rings 91 a and 91 b are used toprevent leakage of hydraulic fluid from the gerotor housing 129.

Rotation of gerotor see 134 created by rotation of input shaft 125creates a suction at inlet 136. The hydraulic fluid drawn into gerotorset 134 is then forced into auxiliary outlet passage 92 under pressureto an implement circuit. The implement circuit returns hydraulic fluidvia the auxiliary inlet passage 93, and then into gerotor housing 129.The gerotor housing provides a path for the hydraulic fluid that isconnected to fluid passage 131 in the porting plate, and thus to gallery137. The path that returns fluid from the implement circuit is notcritical, and may be implemented in a variety of configurations that arewell known in the art. The direction of fluid flow is generally shown bythe arrows in FIG. 16.

As shown in FIG. 17, auxiliary pump cover 90 also includes a chargerelief valve 94 and a check valve 95. These valves can be implemented ina variety of configurations that are well known in the art, and are notunique to this invention. As the FIG. 17 hydraulic schematic indicates,charge relief 94 acts to maintain hydraulic pressure in gallery 137.Excess fluid not demanded by check valves 139 a and 139 b is divertedthrough charge relief valve 94 and returned to gerotor set 134, where itis again routed through auxiliary outlet passage to the implementcircuit 89. If either the motor or pump requires more hydraulic fluidthan is available though implement circuit 89, then the suction causedin gallery 137 when either check valve 139 a and 139 b opens will causecheck valve 95 to open to provide The additional hydraulic fluidrequired.

Another embodiment shown in FIGS. 21-22 uses two completely separategerotor sets 98 and 134 to serve as auxiliary and charge pumps,respectively. In this embodiment, the charge and auxiliary circuits areindependent of each other, as shown in the hydraulic schematic in FIG.21. An implementation of this embodiment is shown in FIG. 22, whereinput shaft 25 has been lengthened to extend through a modified chargecover 129, into the auxiliary pump 98. The operation of this embodimentfollows the description above until return to the auxiliary cover 90,where flow is directed back into the line that leads to reservoir 50 orsump 132. The operation of the charge pump circuit follows thedescription provided in the charge pump section above.

Turning now to FIGS. 23-26, there is illustrated two further embodimentsof the present invention. In particular, both of these embodimentsinclude a charge pump, comprising a gerotor 34 and gerotor housing 29,which is externally mounted to a lower housing cap 200. The lowerhousing cap 200 is, in turn, mounted over an opening in a housingsection 202. As described previously, the gerotor 34 is drivinglyengaged to the pump shaft 25 for creating a quantity of high pressurehydraulic fluid. The high pressure hydraulic fluid is stored in thegallery 37, which is disposed in an area between the center section 26and the charge pump, for use in the hydraulic pump and motor circuit asneeded.

More specifically, the gallery 37 is defined by a manifold 204 which iscooperably positioned between the center section 26 and the housing cap200 such that the upper surface of the manifold 204 matingly engages thecenter section 26, opposite its pump running surface, while the lowersurface of the manifold 204 engages the interior surface of the housingcap 200. Preferably, the manifold 204 is constructed from a generallyresilient and high pressure and heat resistant material, such as moldedplastic or the like, to allow the manifold 204 to be compressionallycaptured between the center section 26 and the housing cap 200. Toprovide further structural integrity to the manifold 204, the manifold204 may be provided with a series of strengthening ribs. Thecompressional capturing of the manifold 204 between the center section26 and the housing cap 200 is desirable as it functions to minimizeleakage from the gallery 37. To further assist in preventing leakage ofthe high pressure fluid as it flows from the charge pump to the gallery,an optional O-ring 205 may be positioned between the manifold 204 andthe housing cap 200 which O-ring 205 seals the fluid flow paththerebetween.

Also preferably supported between the center section 26 and 10 thehousing cap 200 is an oil filter 206 which surrounds the manifold 204.While not required in the preferred embodiment the upper portion of theoil filter housing includes an extension 207 which is trapable betweenthe manifold 204 and the center section 26 when the manifold 204 issecured hereagainst. This cooperation between the housing extension 207center section 26, and manifold 204 functions to further secure the oilfilter 206 in its desired position. A pair of optional O-rings 208, 210are also preferably positioned between the oil filter housing and thecenter section 26 and housing cap 200, respectively, to prevent the flowof hydraulic fluid therebetween.

In a preferred embodiment of the invention, the manifold 204 isconstructed to have a first generally circular upper portion 204A and asecond generally circular lower portion 204B of smaller diameter. It isto be understood that this configuration, of the manifold 204 is notmeant to be limiting and that the manifold 204 may be provided withother geometric arrangements while maintaining its ability to form thegallery 37. Nevertheless, the circular configuration provided to theupper portion 204A is desirable as it is more economical to machine theportion of the center section which preferably mates with the manifold204 to prevent the side to side motion thereof as a circle.Additionally, the differing diameters of the upper portion 204A and thelower portion 204B is preferred as it creates a storage area forunpressurized, filtered fluid which area is in fluid flow communicationwith the charge pump.

During operation, filtered hydraulic fluid will be drawn into the chargepump through an inlet hydraulic passage 212 formed in the lower housing200. The resulting pressurized hydraulic fluid, created by the action ofthe charge pump, will be forced into the gallery 37 through an outlethydraulic passage 214 formed in the lower housing 200 and an opening 216formed in the manifold 204. Check valves 39 a, 39 b mounted within thecenter section 26, operatively connect the gallery 37 and the highpressure hydraulic fluid stored therein with the hydraulic portingformed within the center section 26.

A charge relief valve 35 may optionally be provided to allow pressurizedhydraulic fluid to be dumped from the gallery 37. In particular, thecharge relief valve 35 may be maintained in the manifold 204 (FIGS.23-25), in the center section 26 (FIG. 26), in the housing cap 200, orin the charge cover 29 (not shown). When the charge relief valve 35 ismaintained in the manifold 204, it is preferred that the valve body 218be formed integrally with manifold 204 itself, thus obtaining a valvebody 218 at no additional cost within the manifold 204. This embodimentis particularly desirable since the elimination of the valve body alsoreduces the number or parts and simplifies the assembly process to thepoint where such assembly can occur at an original equipmentmanufacturer. Specifically, the component parts constituting the valvefunction are inserted into this body and retained therein by a retainingring or like type of securing device. Similarly, when the charge reliefvalve 35 is maintained in the center section 26, it is preferred thatthe center section 26 itself by used as the valve body. This embodimentretains the advantages above-described but at a slightly higher costowing to the need to machine the center section 26 to achieve thefeatures required to maintain the charge relief valve 35 therein.

Should the charge relief valve 35 not be utilized, it is preferred thata fixed diameter bleed orifice be formed through the manifold 204.Specifically, the manifold 204 may be provided with an aperture ofpredetermined size which will allow the pressurized hydraulic fluid tobe dumped from the gallery 37 at a rate dependent upon the viscositythereof. Alternatively, while not preferred, the manifold 204 could becaptured between the center section and the charge pump with animperfect seal whereby the pressurized hydraulic fluid may escape fromthe gallery 37 therethrough.

While the embodiments of the invention illustrated in FIGS. 23 and 26have been shown with an externally mounted charge pump, it is alsocontemplated that the charge pump could be internally mounted within thehousing without departing from the spirit of the invention. Accordingly,in a further embodiment, it is contemplated that the charge pump couldbe mounted to the internal surface of the housing cap with the manifoldbeing cooperably disposed between the center section and charge pumpcover.

It is to be understood that the above description should not be read aslimiting the scope of this invention, as further features and benefitswill be obvious to one skilled in the art. This invention should be readas limited by the claims only.

What is claimed is:
 1. A hydrostatic transmission, comprising: a housingforming a sump containing hydraulic fluid, said housing having anopening formed therein; a cap plate having an inner surface and an outersurface and secured to said housing to cover said opening; a centersection having hydraulic porting formed therein mounted in said housing;a hydraulic pump rotatably mounted on said center section; a hydraulicmotor rotatably mounted on said center section and drivingly connectedto said hydraulic pump through said hydraulic porting; a charge pumpmounted on said cap plate, said charge pump in flow communication withsaid sump; and a manifold secured between said inner surface of said capplate and a lower surface of said center section, defining a gallery foruse in storing a quantity of said hydraulic fluid, said gallery being inflow communication with said charge pump and said hydraulic porting. 2.A hydrostatic transaxle for transmitting a motive force created by avehicle engine to an axle, comprising: a housing having an openingformed therein; a cap plate secured to said housing to cover saidopening, said housing and said cap plate forming a sump containinghydraulic fluid; a center section mounted within said housing and havinghydraulic porting formed therein; a pump rotatably mounted on saidcenter section having an input shaft drivingly linked to said vehicleengine; a motor rotatably mounted on said center section andhydraulically connected through said hydraulic porting to said pump,said motor having an output shaft drivingly linked to said axle; amanifold, secured between said center section and said cap plate,defining a gallery for use in storing a quantity of said hydraulicfluid, said gallery being in flow communication with said hydraulicporting; and a charge pump mounted to said cap plate and drivinglylinked to said input shaft, said charge pump being in flow communicationwith said sump and said gallery.